Effective cleaning of rust stained marble
Calcareous materials, like marble used in connection with cultural heritage objects such as statues and pedestals, or as wall facings on buildings, often show a brownish staining owing to contact with iron metal or iron-containing minerals in the stone. The discolouration alters the appearance of the stone, which is undesirable from an aesthetic point of view. Despite rust staining being a conspicuous phenomenon and numerous works that have dealt with the problem of removing rust stains, a simple and non-toxic method has so far been missing. This paper describes a highly efficient method for cleaning rust stains from marble by introducing the chelating amino acid cysteine in a Laponite poultice in combination with the strong reducing agent sodium dithionite.
Cleaning experiments were performed on artificially discoloured samples of various types of Carrara Bianco marble and on naturally rust stained marble. To begin with, solutions of cysteine in combination with sodium dithionite and ammonium carbonate were tested by immersion of samples into the different solutions. Secondly, solutions of cysteine and sodium dithionite with and without buffering were used in a poultice consisting of Laponite® RD, Arbocel® BC1000 and CMC. The poultice was applied on three different marble types: Carrara Fabricotti, Carrara Vagli and Carrara La Piana. Thirdly, the optimized method was tested on original rust stained material of luxury marble, which has been used as wall facing, and finally in situ in Copenhagen on a larger area of The Marble Church showing rust stains due to pyrite oxidation. The cleaning results were evaluated by visual observations, cross sections, and etching of the surface by testing on high gloss marble.
Cleaning of iron-discoloured marble surfaces has been investigated and a new method for removal of rust stained marble has been developed. A solution of 0.1 M cysteine and 0.1 M sodium dithionite in a poultice consisting of Laponite® RD/Arbocel® BC1000/CMC = 10:10:1 has shown to be a fast, simple, cheap, and non-toxic, do-it-yourself method.
Since ancient times, white marble has been used as a popular material for sculptural artefacts such as statues, busts, and friezes as well as an architectural building material with numerous applications from flooring, wall facings, and pedestals, to columns and fountains. Although marble is a relatively stable material, the desired white surface is unfortunately prone to tarnishing when used in outdoor environments . One of the major sources of tarnishing is iron. In addition to the oxidation of internal iron compounds present in stone like pyrite (FeS2) and siderite (FeCO3) [1, 2], contact with iron-rich ground water when Full Body Marble is used in, for example, garden fountains, results in severe and unsightly discolouration . Another cause is the proximity to iron metal, which is oxidized by air in the presence of rain. The solubilized ions are then transported by rain onto the marble surface, resulting in rust formation .
In this study, we have aimed to investigate and develop a new method for rust cleaning of simm marble. The focus has been on the use of cheap and commercially available chemicals. Another target was reduction of Fe(III) to Fe(II) while cleaning. Efficient removal of a slightly soluble material requires a ligand having an overall stability constant comparable to the reciprocal value of the solubility product in order to achieve a favourable equilibrium constant. Based on the solubility product of goethite, efficient removal of rust in Fe(III) stage requires a ligand having a stability constant approaching 1041, whereas removal of Fe(OH)2 only requires a stability constant of 1014. Additionally, the ligand should possess low affinity towards Ca(II) to prevent dissolution of calcite.
Table 1 shows the stability constants of the marble constituents Ca(II), Mg(II), Fe(II) and Fe(III), with the commonly used ligands for rust cleaning i.e. citrate , oxalate , tartrate , edta , tpen [26, 27] and thioglycolate [24, 28], together with cys [21, 28]. The solubility products of CaCO3 , MgCO3 , Fe(OH)2 , and FeOOH  are also given. As seen from the constants, only edta shows affinity towards Mg(II) and Ca(II) in an order resulting in serious dissolution of MgCO3 and CaCO3, whereas the remaining ligands display relatively weak binding constants, causing little dissolution of marble itself. The stability constants of cys are similar to the values of thioglycolate, and cys possess very high affinity towards iron(III), which is even higher than for edta. Towards iron(II) the overall stability constant is of an order of magnitude close to the value for tpen, thus making cys an ideal candidate for cleaning of rust stained marble.